EMC Question of the Week: December 26, 2017
Placing a 1kΩ resistor in series with the reset input of a microprocessor,
- has no effect on the voltage required to reset the processor
- can help to prevent inadvertent resets due to coupled noise
- can reduce conducted noise emanating from the processor
- all of the above
Answer
The best answer is "d". The reset input of a processor typically looks like a 5 pF to 10 pF capacitance, therefore adding a series resistance does not change the amplitude of the signal voltage required to reset the processor. On the other hand, the transition times associated with the input can be substantially increased (typically from 10s of picoseconds to several nanoseconds) by adding a series resistor. This prevents high-frequency transient noise (e.g. coupled from ESD or EFT transients) from triggering a reset. Finally, since these inputs are often tied high by default, on-chip power bus noise can appear on these pins and be conducted away from processor. A series resistor can substantially reduce the noise currents conducted onto the trace attached to these pins.
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